1. Field of the Invention
The invention relates to factory-built housing (and possibly other factory built buildings) and may have application to other areas including without limitation modular and other pre-constructed housing and other pre-constructed buildings, and includes methods for construction and transportation of the same.
2. Description of Related Art
Transportable building construction is subject to difficulties in obtaining sufficient longitudinal dimension, while at the same time exhibiting appropriate stiffness resisting primary bending, quality of ride in transport, and overall business economy. Prior technology has attempted numerous different approaches undertaken in the quest to construct an economic transportable building unit and its associated transport system, but none to the satisfaction of the inventors.
Current transportable buildings can be generally categorized into several types: manufactured homes (which are regulated by federal law), frame-on modular homes, frame-off modular homes and other modular buildings. Each of these three types of transportable building construction as a general rule primarily use the same art of steel frame and wheel sub-assembly construction. FIG. 1 provides a side view of a typical steel frame and wheel sub-assembly 50. Typical approaches taken in prior practice have attached wheels for transportation purposes in one of the following scenarios: locating the floor above the axles, embedding the wheels into recesses of the floor or support structure, or using a dolly trailer to carry the building unit. Each of these solutions represents a trade off in terms of cost of equipment versus ride quality (wherein poor ride quality risks cosmetic or structural damage during or prior to delivery). These solutions have focused on transporting the structure using an external support (even if integrated into the building as an under-frame) to provide the basic structural features needed to support the building over the wheels and to span the distance between the wheels. Because that external support is conceived as a single-use (or few-use) transportation support, it is given little consideration as being a value-added structural system for general integrity of the building purposes.
In each of these cases, due to the fact that the building may have to pass under bridges and other obstacles, the building-plus-carriage must remain below maximum road height requirements. The Department of Transportation typically limits oversized loads to a height of around 15 feet (457 cm), but this figure varies depending on factors such as the states being traveled and the routes taken. The limit that constrains the height of transportable buildings is derived by subtracting the carriage height from the legal road limit height. The steel frame and wheel assembly 50 used by the vast majority of transportable buildings carries the bottom of the floor joists at a height of approximately 36 inches (91 cm) above the ground, leaving only 12 feet (366 cm) for the remainder of the living quarters and the roof. This height constraint proves to be an important factor in the design of transportable buildings and the associated transport system. One implication of this height limit is that many transportable buildings have relatively low pitch roofs or hinged roofs, which may be undesirable for any number of reasons. Likewise, to maximize the height available for the building structure, transportation carriages for transportable buildings frequently use the smallest diameter wheels 51 possible, the lowest profile spring suspension (not shown), and the lowest profile (least tall) I-beams 52 for the frame, each of which may result in a frame and wheel suspension system 50 that does not provide a high quality ride during transport. The result is that a building, constructed in accordance with conventional practice, when transported may be subjected to an undesired level of shock and vibration, risking any number of types of damage, such as crushed and buckled floor panels, cracked sheet rock and torn sheet rock joints, detachment of cabinets, fixtures and trim work.
An additional issue posed by reduction of transport profile is that obstacles such as railroad tracks and uneven ground may cause jolting or impassable areas. Some low boy trailers can elevate the front end of the assembly up to several feet. However, the rear suspension unit on low boy trailers typically has a very limited lifting ability. Those equipped with air ride suspension can typically lift the load approximately eight (8) inches (20 cm), but this may be insufficient when encountering larger obstacles or road variance.
The steel frame and wheel sub-assemblies 50 can sometimes be reused in the case of frame-off modular construction. In some cases manufacturers will transport the frame/wheel subassembly 50 back to the factory for re-use. As a general rule, the sub-assembly 50 will only be usable for two or three additional trips due to DOT safety requirements and structural integrity issues that arise out of wear and tear. In addition, transport cost back to the factory from the consumer's home site incurs additional cost because the subassembly requires special “wide load” permits that also require escort cars.
Some solutions have sought to strengthen the floor structure of the building in attempts to replace the steel frame 50. These activities focus on strengthening the floor, or the material below the floor, rather than conceiving of the structure of the building as a whole. As a result of this focus on the floor or below-flooring area as being the primary or sole location of structural integrity for lifting and spanning purposes, current transportation designs that lift the building make no attempt to provide the building unit with anything other than a “simple support” or “pin” end condition. These transportation systems may not provide adequate resistance to rotation of the ends of the buildings about the horizontal lateral axis (i.e., an axis drawn side-to-side across the width of the building unit, perpendicular to a straight line of travel). Thus the ends of the buildings tend to rotate about the horizontal lateral axis as the unit is lifted and the unit “sags” in the middle. FIG. 2A shows a side view of an idealized simply-supported beam 60 that may conceptually be considered to represent a building where the floors at the ends are sloped toward the middle. The support on the right can move horizontally and is frictionless so that deflection cannot cause tensile stresses within the beam 60 (mimicking the condition in a building during transport where it is supported only by forward and rearward wheels). The beam 60 is shown deflecting under a uniform distributed loading. The figure provides a level reference line 61 to help show the slope of the beam 60 or floor surface at the ends. The slope of beam surface is greatest at the ends directly above the pin supports. FIG. 2B shows an idealized beam 62 supported and constrained at the ends in a cantilever end condition. The support on the right can move horizontally and is frictionless so that deflection cannot cause tensile stresses within the beam 62. The beam 62 is shown deflecting under a uniform distributed loading. A straight, level line 61 has been added to help highlight how the ends of the beam 62 are constrained so they do not rotate as the beam 62 deflects under loading. These figures help show that the ultimate result of not providing a constraint to resist rotation about the horizontal lateral axis is that the buildings are subjected to higher stresses and suffer damage during transport as a result of excessive bending along the length of the unit.
Many conventional manufactured and modular houses require shear walls at selected locations near the middle of the floor plans, to support the exterior and marriage walls and to counter lateral wind forces. They also provide the ceiling and roof truss structure with resistance to uplift and downward forces caused by winds. The requirement for shear walls limits configuration flexibility, such as relocation of or removal of walls.
Manufactured homes and “frame on modular” houses typically have difficulty qualifying for the advantageous financing that is available to site-built homes. Also, these houses tend to suffer from appraisals that assign them at a relatively low value as they are typically treated as depreciating personal property.
Whatever the limitations of transportable buildings, there are undeniable advantages to a building that can be built in a mass production factory environment. These include construction time gauged in hours, rather than months, time-value of money and elimination or reduction of “construction loans” for the consumer, lower costs of construction, economies of scale in purchasing, reduced scrap loss and over-inventory (due to inventory for multiple productions being centralized), reduced exposure and weather damage to materials, use of jigs and fixtures that facilitate assembly of the floor, walls, ceiling, and roof structures, and thereby increase the squareness and uniformity of construction-to-plan, and institutionalized quality control and inspection systems or services in a factory environment.
Use of conventional techniques to join building materials results in transportable buildings not being as strong or as stiff as they could potentially be. In prior practice, wood sheet goods (like plywood and oriented strand board, commonly called OSB) typically are installed with an edge gap of approximately ⅛th inch (3.2 mm) at all four edges. However, in practice, this contributes to the sheet goods each acting to some degree independently, as slippage and movement is allowed between the panels. The intentional provision for such slippage and independent movement ignores the potential value that unifying the panels may have for the overall structure's stiffness.
In keeping with the assumption in prior practice that not only is some measure of independence and movement of components relative to one another acceptable, but is desirable, other joints in the structures typically have been connected in a way that similarly allows for modest amounts of movement among individual components. In site-built housing, where the building is expected to be held in a static condition by its stationary emplacement, this may be acceptable. However, transportable buildings are subjected to additional stresses when they are lifted, hoisted, transported, and otherwise generally subjected to stresses of movement, torsion, bending and shifting. In such conditions, the modest slippage between individual components must be viewed in the aggregate across the structure as a whole. When so viewed with respect to a building spanning a distance greater than 30 feet (914 cm), the aggregate slippage becomes meaningful and translates into substantial slack in the structure. Typical construction in the past of even transportable buildings has relied heavily on fasteners such as nails, staples or screws as the materials to connect the pieces of lumber or sheets of ply material to each other. While these joints do connect the adjoining structural pieces, the joints are insufficiently effective at truly unifying these to act as a single structure. There is therefore, in conventional construction, typically a meaningful amount of slippage between adjoining members, and as a result, structures constructed in this art suffer a large amount of deflection before “the slack is taken out” of these joints.
For example, in the case where plywood sheathing 114 is being fastened to a vertical wall stud 111, the total contact area for a joint secured using adhesive will be 1.5 inches (3.8 cm)×the wall height (say 8 feet=96 inches (244 cm)) equals 144 square inches (929 cm2). In the case where nails are used, assuming that 16 penny nails are used and that nails are spaced 6 inches (15.2 cm) apart, approximately 16 nails will be used. The diameter of a 16 penny bright common nail is approximately 0.162 inches (4.11 mm) and the thickness of plywood sheathing 114 is no greater than 0.72 inches (18.3 mm), so the total bearing surface area of the nail in the plywood 114 is 16*0.162*0.72=1.87 square inches (12.1 cm2). Thus the adhesive joint spreads the applied force over an area approximately 77 times greater than the nail joint delivers. Furthermore, as shear loading is applied to these nails, they will tend to bend in the direction of the applied force, and the effective bearing surface area of the nails will tend to decrease. Moreover, because of the point-contacts created by a nail, screw, staple, or similar fastener, when such fasteners alone are used, when sheathing 114 is subjected to a rotation relative to the substrate (e.g., the studs 111), the panel 114 has some freedom to pivot about the point contact.
Where only slippage-permitting joints are used, rather than a near-zero slip joint, each type of stress—tension, rotation, compression and shear—may allow for meaningful movement across the aggregate of joints before effective transfer of force among structural members.
Consider the joint where the wall bottom plates 121 are fastened to the floor decking 109. In joints constructed using conventional fasteners alone, a loading that results in these members 109, 121 bending will be countered by the sum of resistance provided by these two structures 109, 121 acting independently, not acting in concert. FIG. 2C shows an idealized side view of a composite beam 63 made of four independent beam elements 64 protruding from a wall. FIG. 2D shows how these structures 64 behave and deflect when operating independently. FIG. 2E shows how these structures 64 behave and deflect when unified as one structure 63 and operate together. In order to more clearly demonstrate the concepts, the beams 63, 64 shown in FIGS. 2D and 2E are deflected a certain amount. While the amount of this deflection appears to be equal, no information is given about the loading that would cause such deflection. It is generally true that the unified beam 63 shown in FIG. 2E would require significantly more force to deflect the beam 63 an amount equal to that seen in FIG. 2D. This theoretical example assumes that the beam elements 64 are fabricated to be substantially the same except for the difference of the beam 63 shown in FIG. 2E being unified to act as a united structure. Structures that act independently will neither be as strong nor as stiff as structures that are combined to act as a single unit.
It is now known to the inventors that structures joined by spreading the applied force out over a larger area will tend to form a joint with less slippage than alternate approaches that concentrate the applied forces into smaller areas. Joints formed by spreading the applied force out over a larger area are also very likely to result in joints that are stronger in many respects.
FIG. 3 depicts an idealized graph that contrasts the dashed line 70 depicting the stress-strain (or load-deflection) behavior exhibited by structural elements joined using the conventional art fasteners (such as nails, staples and screws) with the solid line 71 depicting the behavior exhibited by structural elements joined with near-zero slip joints. The leftmost portion 72 of the dashed line 70 depicting current art's stress-strain behavior shows line 70's slope transitioning from being almost flat and increasing to being almost as steep as solid line 71. This area 72 where line 70's slope is less steep than it is on the rightmost portion of the graph indicate a relatively large amount of slippage before “the slack is taken out” and the structure begins to deliver a significant amount of strength and stiffness. The flexibility and post-transport damage exhibited by transportable buildings constructed using conventional practice serve as evidence of this. The upper solid line 71 depicts how a unified structure exhibits no appreciable slippage between components as loading is applied. We use the term “near zero-slip joints” when referencing such joints in which the stiffness of the bonded joint is greater than the stiffness of either of the parent materials being joined. Near zero-slip joints can be formed by substantially or completely covering the surfaces to be joined with a suitable adhesive, the structures to be place are put into place, and the structures are held until the adhesive is cured. Alternatively where the materials are suited for this, mechanical fasteners may be used to hold the assembled joints together while the adhesive sets.
Aggregate slippage is particularly relevant in a structure that will travel roadways because the expected maximum deflection at mid-span must be added to the desired clearance height of the floor during shipment to determine the height at which the floor end supports must be carried. Knowing this height together with the maximum DOT road height limit will allow the designer to select the height of the building unit ends thus determining the roof truss height, which designers may seek to maximize.